test: update 'fixturesDir' refs in a test file

This was a task from Code & Learn at NINA17.

PR-URL: #15824
Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

test/parallel/test-http-url.parse-https.request.js

@@ -2,16 +2,16 @@
 const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
+const { readKey } = require('../common/fixtures');
 
 const assert = require('assert');
 const https = require('https');
 const url = require('url');
-const fs = require('fs');
 
 // https options
 const httpsOptions = {
-  key: fs.readFileSync(`${common.fixturesDir}/keys/agent1-key.pem`),
-  cert: fs.readFileSync(`${common.fixturesDir}/keys/agent1-cert.pem`)
+  key: readKey('agent1-key.pem'),
+  cert: readKey('agent1-cert.pem')
 };
 
 function check(request) {